Subzero temperature fuel and rocket ignition process



United States Patent 3,213,610 SUBZERO TEMPERATURE FUEL AND ROCKETIGNITION PRUCESS John C. Grigger, Oreland, and Henry C. Miller,Hatfield, Pa., assignors to Pennsalt Chemicals Corporation,Philadelphia, lPa., a corporation of Pennsylvania No Drawing. Filed Apr.27, 1962, Ser. No. 190,7918 12 Claims. (Cl. 6035.4)

This invention relates to methods and means useful in rocket propulsionand jet or thrust engines, particularly at subzero temperatures. In oneof its specific aspects, it relates to a hypergolic igniter system. Inanother of its specific aspects, it relates to methods for using saidsystem in the ignition of oxidizer-fuel systems in rocket motors and jetengines.

Hypergolic, or self-igniting, oxidizer-fuel systems are well-known asmeans for producing gaseous products useful as a source of thrust inrocket motors. A hypergolic oxidizer-fuel system chemically consists ofan oxidizing component and a reducing component. The essence of the modeof operation of a hypergolic system is that the individual chemicalcomponents, which alone are not selfigniting in the atmosphere of acombustion chamber, will ignite spontaneously when brought into intimatecontact with each other. The use of such a system, in addition to itseconomic advantages, generally avoids dangers and difficulties resultingfrom ignition system failures which can occur with a system whichdepends on a non-chemical ignition means.

Practically all hypergolic mixtures have delayed ignition periods ofsubstantially less than one second and preferably not more than 50milliseconds at ordinary ambient temperatures at ground level. At lowertemperatures, this ignition delay period becomes longer. At subzerotemperatures, i.e., below zero degrees on the centigrade temperaturescale, the delay can become dangerously prolonged and disastrous throughbuild-up of unignited combustible materials in a combustion chamber.Also, in some cases, complete failure of ignition can result withmaterials which become non-hypergolic in the presence of each other dueto the low temperature.

The temperatures commonly encountered in high altitude operation ofaircraft powered by jet or piston engines is in the range of 40 to 57 C.In outer space, it is expected that temperatures of about -100 C. arecommon. At these extremely low subzero temperatures, the problems ofignition, or re-ignition after a flame-out, of the components even of asystem which is hypergolic at normal ambient temperatures at groundlevel become formidable.

We have now found a hypergolic igniter system which functions usefullyfor starting a fire by auto-ignition at a temperature at least as low asthat of the freezing point of chlorine trifluoride, i.e., at about -83C., and lower, e.g., at about -145 C., in a mixture of halogen fluoridewith perchloryl fluoride (ClO F) or fluorine or a mixture thereof. Wehave found that on bringing together separately and substantiallysimultaneously an oxidizer comprising a halogen fluoride and a metalfuel comprising niobium or tantalum, auto-ignition of the resultingmixture occurs within a few seconds at about 100 C. and in much lessthan a second at higher temperatures, e.g., above around 75 C.

It is known in the prior art that certain metals will ignite atappreciably elevated temperatures, e.g., above about 150 C., in thepresence of halogen fluorides. Thus, uranium will ignite at 150 C., 225C., 250 C., and 260 C. in bromine trifluoride, bromine pentafluoride,chlorine trifluoride (GR) and fluorine, respectively, and zirconiumignites in chlorine trifluoride or fluorine gas at 331mm Patented Get.26, 1965 about 340 C. On the other hand, thorium, aluminum, copper,iron, magnesium and platnium do not ignite in these oxidizers even attemperatures ranging at 410 C. See, for example, Stein and Vogel,Industrial and Engineering and Chemistry, volume 48, pages 418-21(1956). Therefore, it was surprising to find that niobium and tantalumof the Group VA metals ignite spontaneously at subzero temperatures inthe presence of an oxidizer comprising a halogen fluoride, e.g. chlorinetrifluoride. It was also surprising to find that tantalum is morereactive than niobium in this respect.

The Group VA metals are vanadium, niobium and tantalum. Unlike niobiumand tantalum, vanadium does not ignite spontaneously in a halogenfluoride even at room temperature, although vigorous chemical reactionoccurs.

For the practice of the invention, niobium (columbium) and tantalummetal each can be used in commercially pure form. Each of these metalsalso can be used as an alloy consisting substantially of one of thesemetals with the other or with at least one other metal, e.g., iron,zirconium, tungsten or nickel. The metals, or their alloys, can be usedin the form of pellets, preferably of about 0.1 gram in weight orlighter. The metals, or their alloys, also can be used in the form ofshavings, turnings or fine powder. Also, the metals, or their alloys,can be used in the form of a suspension within a liquid fuel. They canalso be used as an ingredient embedded with a binder in the surface ofand within a solidified form of fuel, for example, in an extrudedplastic oxidant-fuel composition containing the niobium or tantalummetal, or an alloy thereof, in pellet, shavings, turnings, powdered orother physical form, alone or in major proportion in combination withanother metal.

The oxidizer which is used in practice of the invention preferably ishalogen fluoride, including chlorine trifluoride, chlorine monofluoride,bromine trifluoride, and bromine pentafluoride. The oxidizer can also behalogen fluoride in combination with fluorine or with perchlorylfluoride, or with a mixture of both of the latter materials. Preferably,the oxidizer is a mixture of halogen fluoride with perchloryl fluoride.In such a mixture, the ratio of perchloryl fluoride to the halogenfluoride can range from about 2421 to 1:5 by weight, more perchlorylfluoride being required to keep the oxidizer in fluid form as thetemperature of use is lowered. A mixture of oxidizer containing fromabout 25% to about 50% by weight of perchloryl fluoride in the halogenfluoride is preferred. Mixtures of perchloryl fluoride with halogenfluoride which are hypergolic to certain fuels at ordinary ambienttemperatures are disclosed and claimed in Gall, US. Patent 3,066,058. Amixture of halogen fluoride with fluorine in similar proportions can beused. Also, fluorine and perchloryl fluoride can be mixed and themixture used with a halogen fluoride, e.g., chlorine trifluoride, insimilar proportions as when perchloryl fluoride is used alone with thehalogen fluoride oxidizer.

In practicing the invention in accordance with one embodiment in rocketpropulsion, chlorine trifluoride and tantalum in the form of fine shotare separately and simultaneously charged into the combustion chamber ofa rocket motor in a way which will be obvious to one skilled in the artsuch that they immediately come into intimate contact, therebyspontaneously igniting. For example, the chlorine trifluoride is pumpedinto the chamber and the shot is blown in with a combustible gaseousmaterial. The gaseous reaction products of combustion are dischargedthrough the exit orifice of said motor to propel the rocket.

The halogen fluorides have little or no vapor pressure at 0 C.Therefore, auxiliary means are usually necessary to charge the halogenfluoride to the combustion chamber. A pump can be used for this purposeif means for driving the pump are available at the subzero temperature.Preferably, however, since pumping means could become inoperative, thehalogen fluoride is pressurized by means of a gaseous material to apressure above that of the combustion chamber. An inert gas, e.g.,nitrogen or helium, can be used for this purpose. However, in some casesit is preferred to use a reactive gaseous material, e.g., perchlorylfluoride or fluorine. Also, since in some cases the temperature of useof the halogen fluoride may be below the freezing point of the halogenfluoride, e.g., below 83 C. in the case of chlorine trifluoride, anauxiliary liquified gaseous ma terial, such as one of the above group,provides a fluid medium in which the halogen fluoride can be chargedinto the combustion chamber in the form of a slurry of crystals and thusbrought into contact with the metal component of our igniter system,e.g., tantalum in powdered form. Despite the fact that the halogenfluoride is in crystalline form, when it is brought into contact withour metal fuel, the reactivity of the metal with the halogen fluoride issufliciently vigorous soon to cause spontaneous ignition of the metal.On ignition, the auxiliary material, e.g., perchloryl fluoride, alsosupports combustion of the metal. Therefore, the oxidizer of ourinvention can be charged to a combustion chamber at extremely lowsubzero temperatures Without need for auxiliary mechanical means.

A metal component of our igniter mixture, e.g., niobium, also can becharged to the combustion chamber without need for auxiliary mechanicalmeans by fluidizing a powder of the metal with a gas, e.g., carbonmonoxide, which is non-reactive with the metal in its powdered form.Accordingly, by means of our invention, positive starting of a fire inthe combustion chamber of a rocket motor or of a jet engine can beachieved even at extremely low subzero temperatures without depending onmechanical means for charging the components of the igniter mixture tothe combustion chamber.

In another embodiment, the hypergolic igniter system of this inventioncan be used as an igniter system for a separate propellant system whichis to provide thrust in a rocket motor. In such use, an amount of metalfrom about 0.1 gram to 1.0 gram in wire or particle form is generallyadequate to provide sufficient heat and flame on spontaneous ignition inan oxidizer comprising halogen fluoride to ignite the thrust-providingpropellant system. The propellant can be in solid form, for example,such as that disclosed and claimed in Guth, US. 2,963,356, butcontaining, in addition to the ingredients of the Guth type ofpropellant, an effective amount of tantalum, niobium or an alloy ofeither in divided form dispersed in the propellant and embedded in thesurface thereof so that it is readily accessible to our oxidizer. Theamount of niobium or tantalum metal or alloy thereof in the propellantcan be from 0.5 to 10 parts per 100 parts by weight of propellant. Forexample, a stream of oxidizer of this invention, e.g., chlorinetrifluoride, is directed against the exposed surface of a solidpropellant containing an alloy of niobium in the combustion chamber of arocket motor. Upon contact with the chlorine trifluoride, hypergolicignition of the niobium alloy is achieved even at temperatures of aboutl C. When the combustion of the propellant becomes selfsustaining, theflow of the oxidizer of our hypergolic ignition system can bediscontinued. The products of combustion are discharged through the exitorifice of the motor to propel the rocket.

In the event that the thrust-providing propellant system comprises aliquid or powdered fuel in combination with a liquid or powderedoxidizer in combustible proportions, the hypergolic igniter system ofthis invention can be used to achieve positive ignition of such apropellant system at subzero temperatures. For example, an oxidizeruseful in the practice of our invention, e.g. chlorine trifluoridecontaining 50\% by weight of perchloryl fluoride, and pellets ofcommercially pure tantalum of about 0.1" diameter are charged separatelyand substantially simultaneously into the combustion chamber of a rocketmotor so that the oxidizer envelops the pellets, thereby causingspontaneous ignition of the tantalum. Separately and substantiallysimultaneously, a thrust-providing propellant mixture consisting ofanother fuel, e.g., kerosene (]P4), and another oxidizer, e.g., oxygen,are charged into the combustion chamber. The latter propellant mixtureignites in the products of combustion of the hypergolic igniter systemof this invention. After the combustion of the thrustprovidingpropellant mixture becomes self-sustaining, the

flow of the components of our hypergolic igniter system can bediscontinued. The gaseous products of reaction are discharged throughthe exit orifice of the rocket motor to propel the rocket.

In view of the reactivity of niobium and tantalum metal and their alloysin halogen fluoride, use of these metals as materials of construction inthe combustion chamber must be avoided. However, this fact presents nogreat problem because other metals, particularly those forming aprotective fluoride film, e.g., Monel or nickel or 18-8 stainless steelcan be used in the combustion chamber where the halogen fluoride ispresent.

The niobium or tantalum metal or one of their alloys can also bedispersed as a stream of fine fluidized powder into the path of a liquidor of a powdered solid fuel which is to be burned in the combustionchamber solely with an oxidizer useful in the practice of this inventionto produce the thrust for operation of the rocket motor. The mixedstream of metal, e.g., niobium, and fuel, e.g., powdered coal fluidizedwith carbon dioxide gas, ignites spontaneously in a stream of anoxidizer of this invention, e.g., chlorine trifluoride, at a subzerotemperature, e.g. in the range of about -1 to 83 C.

Similarly, the mixed stream of our metal fuel and another fuel, e.g.unsymmetrical dimethyl hydrazine (UDMH) or hydrogen, can be used with anoxidizer comprising halogen fluoride, e.g., chlorine trifluoride, toprovide intermittent bursts of thrust in vector rockets attached to aspace vehicle. Thus, positive ignition of the vector rockets atextremely subzero temperatures, e.g., C. and lower, is made possible bythe practice of our invention.

The same technique as described in the preceding paragraph can beemployed also in the ignition of jet engine motors using J P4 fuel atordinary ambient temperatures, as well as at subzero temperatures, inorder to overcome the dangers arising from flame-out in commercialairliner engines during takeoff or in flight.

Many flame-propagating materials will rapidly burn, after ignition bythe hypergolic igniter system of this invention, at a subzerotemperature, in the presence of an oxidizer of the system. Thesematerials include all materials which burn at an elevated temperature inan atmosphere containing halogen or oxygen and halogen, but which maynot ignite spontaneously with a halogen fluoride at a subzerotemperature. Examples of such combustible materials are hydrogen;ammonia; hydra- Zines; carbon compounds, particularly alkyl hydrazines,e.g., symmetrical and unsymmetrical dimethyl hydrazines, arylhydrazines, alcohols, mercaptans, ketones, ethers and hydrocarbons,e.g., acetylene, ethane, propane, hexane and kerosenes; carbonaceousmaterials of all kinds, e.g., wood, coal, coke, carbon, graphite,cellulosic fibers, and synthetic polymers; alkali metals, e.g., lithium,sodium and potassium; alkaline earth metals, e.g., beryllium, magnesium,calcium, strontium and barium; aluminum; iron; phosphorus and its lowervalence compounds; sulfur; boron; silicon; and hydrides and alkyls ofthe listed chemical elements. After any of these flame-propagatingmaterials has been ignited at subzero temperatures by means of ourhypergolic igniter system,

for example as disclosed in any of the preceding embodiments, theoxidizer of our system comprising halogen fluoride can be used alonewith the above material to maintain a flame and to produce products ofcombustion It will be obvious to those skilled in the art that manymodifications may be made within the scope of the present inventionWithout departing from the scope and spirit thereof, and the inventionincludes all such modificauseful in operation of thrust devices. 5tions.

Table I Ex- Periodic ample Metal Table Oxidizer Ignition TemperatureResults and Comments No. Group 1 Niobium, 3 x 5 x VA ClFa, 4 g Aboveliquid nitrogen tempera- Violent, incendiary reaction and bright ture,but below MP. of 011%. bluish-white flame observed before f01F:i wasobserved to melt. Tube use 2 do VA OIFa, 2 g., (31031, 2 g- About 148 FDuring air-warming of tube from liquid nitrogen temperature, white flashof light was observed, followed after a few seconds by bright flamelasting for about seconds. Some fusion of tube. N0 biobium left.

3 do VA Fluorine, 4g N o ignition Liquidfluorineboiledoftatroomtemperature. N o combustion. Metal gained 0.0007 g.weight over original 0.2075 g.

4 Tantalum x x 34... VA .ClFa, 4 g Slightly above liquid nitrogenViolent ignition observed as with temperature. niobium, but earlier.

5 Vanadium, 0.218 g. in fine VA ClFa, 7.6 g No ignition Vigorousbubbling reaction began on granular form. melting of 01113 and continueduntil all ClFa had boiled 011 while warming to about 55 F.

6 Bismuth, 0.024 wire VB 011%, 6.6 g.-. do Liquid 01F; boiled off atroom temperature. No combustion. Metal gained 0.0002 g. over original0.2591 g.

Zirconium IVA ClFa. Liquid ClF; boiled oif at room temperature. N0combustion. Tungsten VIA ClF3 Do.

This invention is further illustrated by Examples 1-8 We claim:

shown in Table I. In carrying out the demonstration of our invention asdisclosed by the examples, a translucent test tube A1. diameter by 6"long, made of polychlorotrifluoroethylene was used as a vessel forholding liquid oxidizer. The test tube was suspended in and cooled in aliquid nitrogen bath (-196 C.). A small piece of metal was placed in thetest tube and cooled to the temperature of the bath. Oxidizer was thenquickly condensed into the chilled test tube, the oxidizer cover ing thechilled metal test piece and freezing instantly to the temperature ofthe bath. The liquid nitrogen bath was then removed and the tube allowedto stand at room temperature. The contents of the test tube wereobserved as the oxidizer warmed. All test work was conducted behindsafety glass barricades in a vented hood.

The results, as shown in Table I, show that, of the metals tested, onlyniobium and tantalum ignite in chlorine trifluoride at subzerotemperatures. The results also show that fluorine does not igniteniobium in the absence of chlorine trifluoride even at roomtemperatures. Further, the results show that neither a Group IVA nor aGroup VIA metal nor a Group VB non-metal element ignites even at roomtemperature in chlorine trifluoride. Also, the results show that achlorine trichloride-perchloryl fluoride oxidizer mixture ignites belowthe freezing point of chlorine trifluoride (F.P. -83.5 C.). The results,taken in view of the known art, accordingly amply demonstrate theunobvious and unexpected ignition properties of the hypergolic ignitersystem of the invention.

Although the practice of the invention has been described principally inconnection with its use in rocket engines at subzero temperatures, theinvention can also be practiced at above zero temperatures wherever itis desired to obtain positive ignition of a combustible material, usingthe hypergolic igniter system of this invention to ensure such ignition.Thus, our hypergolic igniter system can be used in chemical cuttingtools or devices such as those described in Sweetman, US. 2,918,125 andGall, above, by using the niobium or tantalum metal or their alloys ofour igniter system in combination with the halogen fluoride of Sweetmanor the halogen-perchloryl fluoride mixture of Gall.

1. An oxidizer-fuel propellant system which is selfigniting at a subzerotemperature when the oxidizer and fuel are brought in contact with eachother said oxidizer being selected from the group consisting of (a)halogen fluoride and (b) a mixture of halogen fluoride With at least onemember of the group consisting of fluorine and perchloryl fluoride, saidmixture containing at least 4% by weight of halogen fluoride; said fuelconsisting of (a) from about 0.5 to about 10 parts by weight of adivided form of metal selected from the group consisting essentially ofniobium, tantalum, an alloy of niobium with tantalum, an alloyconsisting substantially of niobium, and an alloy consistingsubstantially of tantalum in combination with (b) parts by weight ofother material combustible in said oxidizer.

2. The propellant according to claim 1 in which the metal is niobium.

3. The propellant according to claim 1 in which the metal is tantalum.

4. The propellant according to claim 1 in which the oxidizer is chlorinetrifluoride.

5. A method for initiating rocket propulsion at a subzero temperaturewhich method comprises (a) bringing together separately andsubstantially simultaneously into the combustion chamber of a rocketmotor a hypergolic igniter system comprising an oxidizer selected fromthe group consisting of halogen fluoride and a mixture of halogenfluoride with at least one member of the group consisting of fluorineand perchloryl fluoride, said mixture containing at least about 4% byweight of halogen fluoride, and a fuel comprising a metal selected fromthe group consisting essentially of niobium, tantalum and their alloyswith each other and at least one other metal, thereby causingspontaneous ignition of said metal in said oxidizer; (b) substantiallysimultaneously providing in said combustion chamber a propellant mixtureconsisting of other fuel and oxidizer therefor in combustibleproportions; (c) mixing the combustion products of said hypergolicigniter system with said propellant mixture in the combustion chamber,thereby causing ignition of said propellant mixture; and (d) dischargingthe gaseous reaction products through the exit orifice of said motor.

6. A method according to claim 5 in which the metal is niobium.

7. A method according to claim 5 in which the metal is tantalum.

8. A method according to claim 5 in which the oxidizer is chlorinetrifluoride.

9. A method for starting a fire at a subzero temperature byauto-ignition which method comprises bringing together separately andsubstantially simultaneously an oxidizer comprising halogen fluoride anda fuel comprising at least 0.5 part per 100 .parts of fuel of a dividedform of a metal selected from the group consisting essentially ofniobium, tantalum and their alloys with each other and with at least oneother metal, thereby causing a fire by spontaneous ignition of saidmetal in said oxidizer.

10. The method according to claim 9 in which the met l is n bium '8 11.The method according to claim 9 in which the metal is tantalum.

12. The method according to claim 9 in which the oxidizer is chlorinetrifluoride.

5 References Cited by the Examiner UNITED STATES PATENTS 2,974,484 3/61Cooley 60-354 X 10 OTHER REFERENCES Rosenberg et al., Ind. & Eng. Chem,vol. 45, No. 10, pp. 2283-86, October 1953.

Lang, Handbook of Chemistry, 1946, pp. 58-9.

15 CARL D. QUARFORT-H, Primary Examiner.

LEON D. ROSDOL, Examiner.

5. A METHOD FOR INITIATING ROCKET PROPULSION AT A SUBZERO TEMPERATUREWHICH METHOD COMPRISES (A) BRINGING TOGETHER SEPARATELY ANDSUBSTANTIALLY SIMUTANEOUSLY INTO THE COMBUSTION CHAMBER OF A ROCKETMOTOR A HYPERGOLIC IGNITER SYSTEM COMPRISING AN OXIDIZER SELECTED FROTHE GROUP CONSISTING OF HALOGEN FLUORIDE AND A MIXTURE OF HALOGENFLUORIDE WITH AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF FLUORINEAND PERCHLORYL FLUORIDE, SAID MIXTURE CONTAINING AT LEAST ABOUT 4% BYWEIGHT OF HALOGEN FLUORIDE, AND A FUEL COMPRISING A METAL SELECTED FROMTHE GROUP CONSISTING ESSENTIALLY OF NIOBIUM, TANTALUM AND THEIR ALLOYSWITH EACH OTHER AND AT LEAST ONE OTHER METAL, THEREBY CAUSINGSPONTANEROUS IGNITION OF SAID METAL IN SAID OXIDIZER; (B) SUBSTANTIALLYSIMULTANEOUSLY PROVIDING IN SAID COMBUSTION CHAMBER A PROPELLANT MIXTURECONSISTING OF OTHER FUEL AND OSIDIZER THEREFOR IN COMBUSTIBLEPROPORTIONS; (C) MIXING THE COMBUSTION PRODUCTS OF SAID HYPERGOLICIGNITER SYSTEM WITH SAID PROPELLANT MIXTURE IN THE COMBUSTION CHAMBER,THEREBY CAUSING IGNITION OF SAID PROPELLANT MIXTURE; AND (D) DISCHARGINGTHE GASEOUS REACTION PRODUCTS THROUGH THE EXIT ORIFICE OF SAID MOTOR.